1. Field of the Invention
The present invention concerns micropositioning systems and, more particularly, a micropositioning system for a robotic arm.
2. Description of Prior Art
Robotic devices are finding an increaisng number of applications in a variety of industries. In automotive assembly lines, for example, robotic devices are now used to perform various welding operations. A number of assembly operations are also now performed by robotic devices, such as the assembly of printed card type electronic circuits and even wristwatches. Robotic devices are also employed in the transport of fairly massive workpieces in foundries and the like, typically feeding and removing workpieces from various types of metal forging equipment.
Robotic operations of the type described above usually require a high degree of positional accuracy. In automotive assembly line welding, for example, robotic welds are typically required to be within ten to fifteen thousandths of an inch of a desired weld location. Electronic circuit and wristwatch assembly operations usually require workpiece placement by the robotic equipment to within one to five thousandths of an inch of a desired position. Foundry operations generally require robotic accuracy of approximately fifty thousandths of an inch.
Once common method for achieving positional accuracy with robotic equipment is by the measurement of relative angles between various portions of the robotic structure. Robotic equipment employed in the type of work described above typically have a working implement (e.g. welding tips, workpiece grasping elements, etc.) pivotally attached to an arm structure which is in turn pivotaly atached to a base structure. Given the angles between the arm, base, and working implement, along with the location of the base structure, the position of the working implement can be fairly precisely determined. These positional calculations, however, are still subject to certain inaccuracies. The weight of the workpiece held by the robotic device or the weight of the working implement itself may, for example, cause the robotic arm to deflect. These deflections can cause an offset in the position of the working implement without affecting the relative angles between the various robotic elements.
One common approach to avoiding deflection inducted positional inaccuracies is to simply build stronger and more massive robotic arms. This approach, however, suffers from several disadvantages. Even more massive robotic arms are still subject to a certain amount of deflection, thus setting an upper limit on the positional accuracy available through this approach. Further, the weight of an arm sufficiently massive to achieve a desired positional accuracy may be prohibitive for certain robotic applications. It would, for example, be impractical to use this approach in building a precsion robotic transport device having substantial mobility and the capability of handling loads weighing several thousand pounds with a positional accuracy on the order of a few thousandths of an inch. The weight of a robotic arm sufficiently massive to avoid deflection induced inaccuracies within the desired tolerances would severaly restrict the mobility of the resulting robotic device.
Another approach to avoiding deflecting induced positional inaccuracies is to measure or calculate the spring constant of a robotic arm structure and program this information into a "lookup table" computer memory. Sensors are then attached to the robotic arm to measure the strain on the arm or, alternatively, the weight of the load being lifted by the arm. A theorectical deflection can then be obtained from the "lookup table" memory to offset inaccurate positional information derived from the relative angles between the elements of the robotic structure. This approach, however, fails to account for variations in the robotic arm spring constant resulting from metal fatigue, stress hysteresis, and similar effects. Changes in the robotic structure spring constant will cause a discrepancy between the calculated deflection and the actual structural deflection, resulting in positional miscalculation and inaccuracy.
Thus, there still exists a need for a robotic micropositioning system which can compensate for deflection induced positional inaccuracies with greater accuracy then presently available through "lookup table" deflection calculations without resorting to prohibitively massive robotic structures.